Recently, optical discs have been drawing attention as exchangeable media with a high capacity and an optical disc unit is expected to be used more widely as a video recorder in the future. One of the reasons why the optical disc is a high-capacity exchangeable medium is that it allows performance of recording/reproduction without contacting the medium. In other words, a laser beam is focused and an information recording layer of an optical disc is irradiated with the focal point thereof, and thus recording and reproduction are performed. Therefore, even when some dirt or dust is adhered on the surface of the disc, a head crash does not occur as it does, for example, in magnetic recording.
However, such a characteristic of the optical disc is based on a major premise: a focus control, i.e., a control for maintaining the distance between the focal point of a convergence laser beam and the information recording layer within an allowable error limit is stably performed. Once the focus is out of control, an objective lens actuator of an optical head runs out of control and may crash into the optical disc surface. Such an off-focus frequently occurs particularly when focusing is performed, i.e., during the process of shifting a laser beam focal point into a capture range of the focus control and then closing a focus control loop, immediately after an optical disc drive is activated. Thus, conventionally, various methods of focusing have been studied.
For example, in a method described in Japanese Laid-Open Publication No. 9-115147, whether an initial laser beam focal point is close or far relative to the optical disc information recording layer is determined beforehand and the focusing process is performed in accordance with this initial state. When the focal point is determined to be close relative to the information recording layer, the objective lens actuator is driven to bring the focal point closer to the information recording layer and when the focal point enters the capture range of the focusing, the operation is switched to a close loop operation of the focus control. Alternately, when it is determined to be far, the actuator is driven to be further, and then the operation is similarly switched to the close loop operation.
However, the conventional method has the problem of an increased rate of collision between an objective lens and the optical disc surface in the case where a working distance of the objective lens (the distance between the objective lens and the optical disc surface) is smaller than the wobbling of the optical disc. Specifically, there is no problem when the focusing is achieved with one attempt. However, if it fails, even though the objective lens actuator does not run out of control, in the case where the wobbling of the disc is greater than the working distance, there is a possibility that the disc collides into the objective lens.
For the currently available CD players and DVD players, the NA (numerical aperture) of the objective lens is about 0.45 to 0.6 at most. Thus, it is possible to ensure a working distance of 0.5 mm or more. Therefore, wobbling of about 0.2 mm, which may usually occur, can be sufficiently absorbed. However, if the NA is raised to its limit in order to further increase the recording capacity in the future, the designed working distance of the objective lens becomes extremely small. For example, if the NA is 0.85, even if the thickness of a protective layer is lowered to about 0.1 mm (the thickness of a protective layer is 1.2 mm in a CD and 0.6 mm in a DVD), the working distance is about 0.15 mm.
The present invention was conceived in view of such problems. The objective of the present invention is to provide an optical disc unit for performing a focus control which is capable of significantly reducing the frequency of collisions of an objective lens into an optical disc surface even when a sufficient working distance of the objective lens is not ensured due to an increased capacity of the optical disc.
As a conventional optical disc unit, there is an optical disc unit in which an optical beam generated from a light source such as a semiconductor laser is focused on and applied to an optical disc rotating with a predetermined rotation number and signals recorded on the optical disc are reproduced. The optical disc includes a plurality of tracks formed in spiral forms. The tracks are formed of grooves having concave and convex portions. A recording film of a phase-shift material or the like is attached to an information surface. For recording information on the optical disc, the strength of the optical beam is changed in accordance with the information, with a focus control performed such that a focal point of the optical beam is on the information surface, and with a tracking control performed such that the focal point is on the tracks. Thus, the reflectance of the recording film is varied. For reproducing information on the optical disc, light reflected off the optical disc is received at a photodetector, similarly, with the focus control performed such that a focal point of the optical beam is on the information surface and with the tracking control performed such that the focal point is on the tracks. The output of the photodetector is processed to reproduce the information.
A focus error signal indicating misalignment between an information surface of an optical disc and a focal point of an optical beam is detected by astigmatic method or the like. The focus error signal becomes zero when the focal point is on the information surface. In general, the detection limit of the astigmatic method is about 10 μm. Thus, for operating a focus control system, the objective lens is moved beforehand so as to shift the position of the focal point into the detection limit. The focus control is performed at the time when the focus error signal crosses zero. However, when the focal point passes the optical disc surface, the focus error signal also crosses zero. If the focus control is performed when zero-crossing occurs at the optical disc surface, the focal control is performed such that the focal point is positioned on the optical disc surface. In order to prevent this, the fact that the reflectance of the information surface is higher than that of the optical disc surface is utilized. More specifically, zero-crossing which occurs at the recording surface is detected when the level of the total internal reflection amount is detected to exceed a predetermined level.
Recently, a rewritable optical disc which has two information surfaces on one side of the optical disc has been proposed. Hereinafter, such an optical disc is referred to as a doublelayer optical disc. In the doublelayer optical disc, when information on the information surface further from the objective lens is reproduced, it is reproduced with an optical beam which has been transmitted through the information surface closer to the objective lens. Thus, the doublelayer optical disc is designed such that the reflectance of the information surface closer to the objective lens is low.
Accordingly, the amount of light from each of the information surfaces which is received at a photodetector becomes small. The optical disc having one information surface is referred to as a single-layer optical disc.
As described above, the level of the total internal reflection amount at each of the information surfaces becomes low in the doublelayer optical disc. Thus, it is difficult to distinguish the optical disc surface and the information surfaces based on the total internal reflection amount. This means that it is difficult to ensure the focusing to the information surface if the focusing is performed in a method similar to the conventional method.
The present invention is conceived in view of the above-described problem. The objective of the present invention is to provide an optical disc unit which is capable to ensure the focusing to the information surface even in the case where difference between the amount of the total internal reflection off the optical disc surface and the amount of the total internal reflection off the information surface is small (for example, in the case where a doublelayer optical disc is used).